When we see records being broken and unprecedented events such as this, the onus is on those who deny any connection to climate change to prove their case. Global warming has fundamentally altered the background conditions that give rise to all weather. In the strictest sense, all weather is now connected to climate change. Kevin Trenberth

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Climate Change Research Centre, University of New South Wales, Sydney, New South Wales 2052, Australia

By 1994 all major industrialized nations, including the United States, had ratified the United Nations Framework Convention on Climate Change (UNFCCC), yet 15 years later policymakers still debate how best to formulate emissions legislation.

Article 2 of the UNFCCC calls for ‘‘stabilization of greenhouse gas concentrations . . . at a level that would prevent dangerous anthropogenic interference with the climate system.’’ Emissions targets are commonly quoted as a percentage reduction relative to a baseline year.

A different framework for emissions targets is presented in a recent issue of PNAS (1), wherein the targets are set as a cumulative emissions inventory, spelling out to policymakers the net emissions allowable to avoid the worst impacts of climate change.

Setting emissions targets around a net cumulative quota is a familiar paradigm for policymakers. It is analogous to planning for expenditure against a net income or setting a catch quota to maintain a sustainable fishery. In such cases, the available resource is fundamentally limited in a cumulative sense; harvest or spend too much and things become unsustainable.

For the global harvesting of fossil fuels the message becomes clear: burn beyond a cumulative cap and you commit the planet to a high risk of dangerous anthropogenic climate change.

The article by Zickfeld et al. (1) uses a coupled climate model to carefully diagnose, via inverse methods, the level of emissions allowable to track toward a given stabilization target for globalwarming. Normally, the problem is addressed in reverse: namely, for a given future emission pathway (2), how will the climate system respond? Both approaches are valid, yet to make meaningful projections they each need to carefully incorporate coupled carbon feedbacks.

Carbon feedbacks occur when there are climate-induced changes in the net fluxes of carbon between the land/ocean and the atmosphere. The strength of the feedback depends on the scale of physical climate change and biophysical processes in the ocean and land systems. A simple example of an ocean carbon feedback is caused by the solubility of CO2, which varies inversely with temperature. Ocean warming reduces...